U.S. patent number 9,697,450 [Application Number 15/224,219] was granted by the patent office on 2017-07-04 for magnetic stripe data transmission system and method for reliable data transmission and low power consumption.
This patent grant is currently assigned to ALPHA AND OMEGA SEMICONDUCTOR INCORPORATED. The grantee listed for this patent is Alpha and Omega Semiconductor Incorporated. Invention is credited to Gilbert S. Z. Lee.
United States Patent |
9,697,450 |
Lee |
July 4, 2017 |
Magnetic stripe data transmission system and method for reliable
data transmission and low power consumption
Abstract
A magnetic stripe data transmission (MST) driver and a method
for driving the MST are disclosed. The MST driver is configured to
transmit magnetic strip data comprising of streams of pulses. The
MST driver comprises a pair of high side switches and a pair of low
side switches. The pair of high side switches comprises a first
switch and a second switch. The pair of low side switches comprises
a third switch and a fourth switch. The first, second, third and
fourth switches are arranged in a full bridge type configuration
connected across a voltage source and a ground. An inductive coil
is connected across outputs of the full bridge type configuration
of the switches. The MST driver includes a switch driver configured
to drive the pair of low side switches and the pair of high side
switches under current slope control using pulse width modulation.
The driven load current has a rising portion and a falling portion
through the inductive coil in a forward direction or in a reverse
direction with programmable load current rising and falling slopes
to induce a recognizable back electromagnetic force at a receiver
emulating the magnetic strip data during the load current rising
and falling portions and to reduce power loss during time periods
without signal transmission.
Inventors: |
Lee; Gilbert S. Z. (Saratoga,
CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Alpha and Omega Semiconductor Incorporated |
Sunnyvale |
CA |
US |
|
|
Assignee: |
ALPHA AND OMEGA SEMICONDUCTOR
INCORPORATED (Sunnyvale, CA)
|
Family
ID: |
59152430 |
Appl.
No.: |
15/224,219 |
Filed: |
July 29, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06K
19/06206 (20130101); G06K 19/07345 (20130101); G06K
7/086 (20130101) |
Current International
Class: |
G06K
7/08 (20060101); G06K 19/06 (20060101); G06K
19/073 (20060101) |
Field of
Search: |
;235/449 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Dan Sweet--Cypress Semiconductor Corp.; "Design How-To Building a
Reliable Magnetic Card Reader (Part 1 of 2)"; EE Times--Connecting
the Global Electronics Community--designlines Wireless &
Networking; Jun. 14, 2010--5 pages. cited by applicant.
|
Primary Examiner: Franklin; Jamara
Attorney, Agent or Firm: Lin; Chen-Chi
Claims
The invention claimed is:
1. A magnetic stripe data transmission (MST) driver configured to
transmit magnetic strip data including a stream of pulses, the MST
driver comprising: a pair of high side switches comprising a first
switch and a second switch; a pair of low side switches comprising
a third switch and a fourth switch; the first, second, third and
fourth switches being arranged in a full bridge type configuration
connected across a voltage source and a ground; an inductive coil
connected across outputs of the full bridge type configuration of
the switches; and a switch driver configured to drive the pair of
low side switches and the pair of high side switches so as to
control load current slopes of load current rising and falling
portions of a load current to generate a back electromagnetic force
emulating the magnetic strip data during the load current rising
and falling portions through the inductive coil.
2. The MST driver of claim 1, wherein the switch driver is
configured to drive the pair of low side switches or the pair of
high side switches by selectively and repeatedly switching between
ON states and OFF states so as to control the load current to
generate a selected rising portion and a selected falling portion
through the inductive coil in a forward direction or in a reverse
direction with programmable load current rising and falling slopes
to generate magnetic signals during the load current rising and
falling portions.
3. The MST driver of claim 1, wherein the switch driver generates a
first control signal for reliable data transmission and a second
control signal for power loss reduction.
4. The MST driver of claim 3, wherein the switch driver is
configured to drive the pair of low side switches in a forward
direction by setting the first switch in a continuously ON state
and by repeatedly switching the fourth switch between an ON state
and an OFF state according to the first control signal and then the
second control signal.
5. The MST driver of claim 4, wherein the switch driver is
configured to repeatedly switch the fourth switch between the ON
state and the OFF state according to a duty cycle of the first
control signal and a duty cycle of the second control signal;
wherein the duty cycle of the first control signal is adjusted to
set the rising portions of the load current in the forward
direction not exceeding a predetermined current limit with a
positive first slope so as to induce a negative back
electromagnetic force generating a signal lower than a negative
reference voltage in a receiver to recognize the induced back
electromagnetic force corresponding to a low pulse signal; and
wherein the duty cycle of the second control signal is adjusted to
set the falling portions of the load current in the forward
direction after reaching the predetermined current limit with a
second slope having a value less than a value of the positive first
slope to induce a positive back electromagnetic force generating
another signal lower than a positive reference voltage in the
receiver, to reduce the power loss during time periods without
signal transmission.
6. The MST driver of claim 3, wherein the switch driver is
configured to drive the pair of low side switches in a reverse
direction by setting the second switch in a continuously ON state
and by repeatedly switching the third switch between an ON state
and an OFF state according to the first control signal and then the
second control signal.
7. The MST driver of claim 6, wherein the switch driver is
configured to repeatedly switch the third switch between the ON
state and the OFF state according to a duty cycle of the first
control signal and a duty cycle of the second control signal;
wherein the duty cycle of the first control signal is adjusted to
set the falling portions of the load current in the reverse
direction not exceeding a predetermined current limit with a
negative first slope so as to induce a positive back
electromagnetic force generating a signal higher than a positive
reference voltage in a receiver to recognize the induced back
electromagnetic force corresponding to a high pulse signal; and
wherein the duty cycle of the second control signal is adjusted to
set the rising portions of the load current in the reverse
direction after reaching the predetermined current limit with a
positive second slope having an absolute value less than an
absolute value of the first slope to induce a negative back
electromagnetic force generating another signal higher than a
negative reference voltage in the receiver, to reduce the power
loss during time periods without signal transmission.
8. A method for driving a magnetic stripe data transmission (MST)
driver with low power and with transmission of signals generated by
the MST driver, the method comprising the steps of: providing a
pair of high side switches comprising a first switch and a second
switch; providing a pair of low side switches comprising a third
switch and a fourth switch; the first, second, third and fourth
switches being arranged in a full bridge type configuration
connected across a voltage source and a ground; providing an
inductive coil connected across outputs of the full bridge type
configuration of the switches; generating magnetic signals by
driving a load current having rising portions and falling portions
through the inductive coil in a forward direction or in a reverse
direction with programmable load current rising and falling slopes
to induce a recognizable back electromagnetic force at a receiver
emulating the magnetic strip data during the load current rising
and falling portions and to reduce power loss during time periods
without signal transmission by controlling the programmable load
current rising and falling slopes using pulse width modulation
(PWM).
9. The method of claim 8 further comprising the step of providing
diodes connected across each of the first, second, third and fourth
switches for facilitating free-wheeling of the load current
corresponding to stored energy in the inductive coil during OFF
periods of the switches.
10. The method of claim 8, wherein the step of generating magnetic
signals by driving the load current through the inductive coil with
the programmable load current rising and falling slopes includes
selectively and repeatedly switching of the pair of low side
switches or the pair of high side switches.
11. The method of claim 10, wherein the selectively and repeatedly
switching of the pair of low side switches comprises: setting the
first switch in a continuously ON state and repeatedly switching
the fourth switch between an ON state and an OFF state for driving
the load current in the forward direction with the programmable
load current rising and falling slopes; and setting the second
switch in a continuously ON state and repeatedly switching the
third switch between an ON state and an OFF state for driving the
load current in the reverse direction with the programmable load
current rising and falling slopes.
12. The method of claim 10, wherein the selectively and repeatedly
switching of the pair of high side switches comprises: setting the
fourth switch in a continuously ON state and repeatedly switching
the first switch between an ON state and an OFF state for driving
the load current in the forward direction with the programmable
load current rising and falling slopes; and setting the third
switch in a continuously ON state and repeatedly switching the
second switch between an ON state and an OFF state for driving the
load current in the reverse direction with the programmable load
current rising and falling slopes.
13. The method of claim 8, wherein the step of generating magnetic
signals by driving the load current through the inductive coil in
the forward direction with the programmable load current rising and
falling slopes includes repeatedly switching the switches between
ON and OFF states according to a duty cycle of a first PWM control
signal and a duty cycle of a second PWM control signal comprising:
adjusting the duty cycle of the first PWM control signal to set the
rising portions of the load current in the forward direction not
exceeding a predetermined current limit with a positive first slope
so as to induce a negative back electromagnetic force generating a
signal lower than a negative reference voltage in a receiver to
recognize the induced back electromagnetic force corresponding to a
low pulse signal; and adjusting the duty cycle of the second PWM
control signal to set the falling portions of the load current in
the forward direction after reaching to the predetermined current
limit with a second slope having a value less than a value of the
positive first slope to induce a positive back electromagnetic
force generating another signal lower than a positive reference
voltage in the receiver, to reduce the power loss during the time
periods without signal transmission.
14. The method of claim 13, wherein the positive first slope is
selectively attained to provide sufficient durations of the load
current rising portions to recognize the induced negative back
electromagnetic force generated signal in the receiver as the low
pulse signal.
15. The method of claim 8, wherein the step of generating magnetic
signals by driving the load current through the inductive coil in
the reverse direction with the programmable load current rising and
falling slopes includes repeatedly switching the switches between
ON and OFF states according to a duty cycle of a first PWM control
signal and a duty cycle of a second PWM control signal comprising:
adjusting the duty cycle of the first PWM control signal to set the
falling portions of the load current in the reverse direction not
exceeding a predetermined current limit with a negative first slope
so as to induce a positive back electromagnetic force generating a
signal higher than a positive reference voltage in a receiver to
recognize the induced back electromagnetic force corresponding to a
high pulse signal; adjusting the duty cycle of the second PWM
control signal to set the rising portions of the load current in
the reverse direction after reaching the predetermined current
limit with a positive second slope having an absolute value less
than an absolute value of the first slope to induce a negative back
electromagnetic force generating another signal higher than a
negative reference voltage in the receiver; to reduce the power
loss during the time periods without signal transmission.
16. The method of claim 15, wherein the negative first slope is
selectively attained to provide sufficient durations of the load
current falling portions to recognize the induced positive back
electromagnetic force generated signal in the receiver as the high
pulse signal.
17. The method of claim 8, wherein an intermediate stage is between
the forward direction and the reverse direction and wherein the
load current reduces to zero in the intermediate stage for improved
power efficiency.
18. The method of claim 8, wherein linearity of the load current
rising and falling slopes is controlled by changing a duty cycle of
a PWM control signal; wherein for a linear slope, a PWM duty cycle
is varied and wherein for a non-linear logarithmic slope, the PWM
duty cycle is a constant.
19. The method of claim 8, wherein a PWM switching frequency during
the rising and falling portions is larger than an input signal
frequency to reduce current ripple in the load current.
20. The method of claim 8 further comprising a pulse frequency
modulation method including a constant on-time control and a
constant off-time control to control the load current rising and
falling slopes.
Description
FIELD OF THE INVENTION
This invention relates generally to a system and a method for
directly transmitting magnetic stripe data to ensure reliable
transmission of the magnetic stripe data with low power
consumption. More particularly, the present invention uses a pulse
width modulation in a magnetic stripe data transmission driver. The
method drives switches of the magnetic stripe data transmission
driver to control current slopes of the generated signals in the
magnetic stripe data transmission driver for low power, reliable
data transmission.
BACKGROUND OF THE INVENTION
Magnetic stripe data transmission or magnetic security transmission
(MST) is a technology that magnetic signals similar to magnetic
stripe data of a traditional payment card are transmitted from a
transmitter to a receiver by an MST driver. The transmitter may be
a host device such as a smart-phone. The receiver may be a payment
terminal's card reader. The magnetic signals emulate magnetic
stripe data of the payment card that are normally read by a card
reader while physically swiping the payment card on a reader
head.
FIG. 1 shows a schematic representation of an inscription of
payment card data on a magnetic stripe of the payment card.
Waveforms corresponding to the magnetic stripe data are picked up
by the payment terminal's card reader's head while swiping of the
payment card along with digital equivalent of the waveforms. The
MST driver emitted magnetic signals emulate the same waveforms at
the payment terminal's card reader without swiping of the payment
card.
In conventional magnetic stripe data transmission or magnetic
security transmission (see U.S. Pat. No. 8,814,046), the MST driver
is configured to transmit the magnetic strip data comprising
streams of pulses. The MST driver preferably includes a full bridge
type switch configuration connected across a voltage source and a
ground to drive bidirectional load current through an inductive
coil according to the magnetic strip data. The MST driver transmits
the magnetic signal to the card reader. In the transmission of the
magnetic signal, magnetic flux density of the inductive coil is
varied according to the load current density, inductance value and
the load current slope of the inductive coil which remotely induces
a back electromagnetic force (B.sub.emf) in a receiver of the card
reader. If the back electromagnetic force (B.sub.emf) is higher
than a threshold value, the card reader then recognizes it as a
High pulse. If B.sub.emf is lower than another threshold value,
then the card reader recognizes it as a Low pulse. The High and Low
pulses in combination can re-construct the card reader's read head
waveforms.
FIG. 2A shows a circuit representation of the MST driver. The MST
driver comprises four MST driver switches 101, 102, 103 and 104
arranged in a full bridge type configuration connected across a
voltage source and a ground V.sub.M 108. An MST coil 105 is modeled
by its inductor 106 having inductance L.sub.1 and series resistance
R.sub.1 107. Each of the MST driver switches includes a respective
body diode (D1-D4) connected across said each switch that plays a
role of free-wheeling current path of stored energy in the inductor
106 during a switch off period.
The MST driver switches 101, 102, 103 and 104 are driven by an
external or a built-in driving integrated circuit (IC). They have
pulse shaped driving waveforms with usually 50% duty ratio of a
constant frequency or a doubled frequency. In the MST driver, both
the first 101 and the fourth 104 switches are simultaneously turned
on for driving the load current in the MST coil 105 in a forward
direction. Both the second 102 and the third 103 switches are
simultaneously tuned on for driving the load current in the MST
coil 105 in a reverse direction.
FIG. 2B shows an MST driver's switch driving operation and a
corresponding load current waveform. The waveform can be divided
into 6 time durations, T1, T2, T3, T4, T5 and T6. Time durations
T1, T2 and T3 may be forward driving periods. The load current is
positive during time durations T1, T2 and T3. Time durations T4, T5
and T6 may be reverse driving periods. The load current is negative
during time durations T4, T5 and T6. The positive or negative value
is entirely based on a designer's perspective.
In the FIG. 2B, the first 101 and the fourth 104 switches are
turned on. The load current increases during T1 period. It reaches
a positive peak current. In T3 period, the first 101 and the fourth
104 switches are turned off and then the second 102 and the third
103 switches are turned on. The load current starts decreasing
abruptly but is still positive. It is called a reverse braking.
With the second 102 and the third 103 switches in a turned-on
state, the load current becomes negative in the T4 period. During
T4 period, the load current slope and the absolute peak value are
the same as T1 except that they are in opposite directions and are
negative values. During T5 period, the negative peak current
continues to flow. In T6 period, the second 102 and the third 103
switches are turned off and the first 101 and the fourth 104
switches are turned on. The load current begins to fall abruptly
and has the same slope as T3 except that they are in opposite
directions.
FIG. 2C shows switching cycles of the MST driver switches, the
corresponding load current waveforms in the MST coil and induced
back electromagnetic force (B.sub.emf) at card reader's receiver.
When the first 101 and the fourth 104 switches are driven by the
same signal to turn on, the load current I.sub.L through the MST
coil 105 starts to increase from previous current and reaches the
peak current I.sub.p. The peak current I.sub.p is dependent on a
supply voltage of a voltage source V.sub.M 108 and the total series
resistance R.sub.1 107 of the MST coil. It can be represented
as
##EQU00001## if the switches' on-resistance is ignored. The load
current
.function..times..times. ##EQU00002## increases exponentially with
the power of
##EQU00003## where L.sub.1 is the inductance value of the MST coil.
Similarly, if the second 102 and the third 103 switches are driven
by the same signal to turn on, the load current
.function..times..times. ##EQU00004## through the MST coil 105
starts to decreases exponentially with the power of
##EQU00005## from the previous current and reaches -I.sub.P.
In the FIG. 2C, a first (I) and a second (II) transient instant of
the load current contribute to the magnetic signal transmission,
since the induced B.sub.emf reaches its peak value during transient
variation of the load current depending on the load current slope
in the B.sub.emf waveform of FIG. 2C. The steady state periods of
load current fixed to +I.sub.p or -I.sub.p have no contribution to
induce B.sub.emf. If the induced B.sub.emf generates a voltage
signal higher than a positive threshold voltage V.sub.r on the
receiver in the card reader, the card reader recognizes it as
"High". If the induced B.sub.emf generates a voltage signal lower
than a negative threshold voltage -V.sub.r, the card reader
recognizes it as "Low".
The back electromagnetic force (B.sub.emf) depends on the magnetic
flux density change ratio which follows current density change
ratio in the inductive coil. The current density change ratio to
time is basically the load current slope which is inversely
proportional to the inductive coil's inductance value. In a fast
current slope, the induced B.sub.emf is big. In a slow current
slope, the induced B.sub.emf is small. In a fast current slope, if
the corresponding duration is too short the receiver in the card
reader may not recognize the signal. In a fast current slope with
long duration, a peak inductive current increases. It may exceed
current rating of the MST driver. It causes additional power loss
by high current. The high current slope has side effect, for
example, noise and Electromagnetic Interference (EMI) issues.
Optimization and control of the load current slope and time
duration are important in the MST driving technology so as to
ensure reliable signal transmission while consuming less power.
However, in the conventional MST driver, the load current slope
cannot be controlled except changing parameters including coil's
inductance, series resistance of the coil or on-resistance of the
full bridge driver's switches. It may not be easy to control those
parameters because limiting factors have trade-offs in performance,
cost and form factor. One way is to increase the inductance of the
MST coil, but bigger inductance requires larger size and increased
cost. Therefore, prior art MST driver cannot deliver. The prior art
MST driver has low energy efficiency due to limited inductance. It
requires long duration to achieve good transmission quality. It may
lose signal because of increased efficiency.
The performance of the prior art MST driver is affected by the
power supply voltage and the MST coil because it is difficult to
control or adjust them. In term of efficiency, the prior art method
consumes a lot of power even during time periods without signal
transmission. The signal transmission is done only in the transient
period of the load current. The steady state of the peak current
consumes power without conducting work. It is much longer than the
transient time. Energy efficiency is much worse. It has a big
impact on a power supply system.
It has a need to develop a new MST driver that can program or
control the load current slope value and time durations to ensure
reliable signal transmission with less power consumption.
SUMMARY OF THE INVENTION
In examples of the present disclosure, a magnetic stripe data
transmission (MST) driver is disclosed. The advantages of the MST
driver includes low power consumption and reliable transmission of
magnetic signals.
An MST driver is configured to transmit magnetic strip data
including streams of pulses. The MST driver includes a pair of high
side switches and a pair of low side switches. The pair of high
side switches comprises a first switch and a second switch. The
pair of low side switches comprises a third switch and a fourth
switch. The first, second, third and fourth switches are arranged
in a full bridge type configuration connected across a voltage
source and a ground. An inductive coil connects across outputs of
the full bridge type configuration of the switches.
A switch driver is configured for driving the pair of low side
switches and the pair of high side switches under current slope
control using pulse width modulation for inducing recognizable back
electromagnetic force at a receiver. It emulates the magnetic strip
data during load current rising and falling portions through the
inductive coil.
In examples of the present disclosure, a switch driver of the MST
driver is configured to drive the pair of low side switches or the
pair of high side switches by selectively and repeatedly switching
between an ON state and an OFF state. It drives the load current
including a rising portion and a falling portion through the
inductive coil in a forward direction or in a reverse direction
with programmable load current rising and falling slopes to
generate magnetic signal for inducing the recognizable back
electromagnetic force at the receiver. It emulates the magnetic
strip data during the load current rising and falling portions and
reduces power loss during time periods without signal
transmission.
In examples of the present disclosure, a switch driver of the MST
driver includes pulse width modulator configured to generate a
first pulse width modulation (PWM) control signal for reliable data
transmission and a second PWM control signal for power loss
reduction.
In examples of the present disclosure, a switch driver of the MST
driver is configured to drive the pair of low side switches to
control the load current so as to have the rising portion and the
falling portion through the inductive coil in a forward direction
with the first switch in a continuously ON state and repeatedly
switching the fourth switch between an ON state and an OFF state
according to the first PWM control signal and then the second PWM
control signal.
In examples of the present disclosure, a switch driver of the MST
driver is configured to drive the pair of low side switches to
control the load current so as to have the rising portion and the
falling portion through the inductive coil in a reverse direction
with the second switch in a continuously ON state and repeatedly
switching the third switch between an ON state and an OFF state
according to the first PWM control signal and then the second PWM
control signal.
In examples of the present disclosure, a switch driver of the MST
driver is configured to drive the fourth switch by repeatedly
switching between an ON state and an OFF state according to duty
cycles of the first PWM control signal and the second PWM control
signal. The duty cycle of the first PWM control signal is adjusted
to set a rising portion of the forward load current to a current
limit in a positive first slope to induce a negative back
electromagnetic force lower than a negative reference voltage in
the receiver and to recognize the induced back electromagnetic
force corresponding to a low pulse signal. The duty cycle of the
second PWM control signal is adjusted to set a falling portion of
the forward load current after reaching the current limit in a
second slope opposite to the positive first slope and slower than
the positive first slope value to induce a positive back
electromagnetic force lower than a positive reference voltage in
the receiver, which can be ignored, to reduce the power loss during
the time periods without signal transmission.
In examples of the present disclosure, a switch driver of the MST
driver is configured to drive the third switch by repeatedly
switching between an ON and an OFF state according to the duty
cycles of the first PWM control signal and the second PWM control
signal. The duty cycle of the first PWM control signal is adjusted
to set a falling portion of the reverse load current to the current
limit in a negative first slope to induce a positive back
electromagnetic force higher than the positive reference voltage in
the receiver to recognize the induced back electromagnetic force
corresponding to a high pulse signal. The duty cycle of the second
PWM control signal is adjusted to control a rising portion of the
reverse load current after reaching to the current limit in a
positive second slope slower than the first slope to induce a
negative back electromagnetic force higher than the negative
reference voltage in the receiver, which can be ignored, to reduce
the power loss during the time periods without signal
transmission.
In examples of the present disclosure, a method for driving the MST
driver is disclosed. The advantages of the method includes low
power consumption and reliable transmission of the MST driver
emitted signals. An MST driver is configured to transmit magnetic
strip data comprising of streams of pulses. The MST driver
comprises a pair of high side switches comprising a first switch
and a second switch; and a pair of low side switches comprising a
third switch and a fourth switch. The first, second, third and
fourth switches are arranged in a full bridge type configuration
connected across a voltage source and a ground. An inductive coil
connects across outputs of the full bridge type configuration of
the switches. The method generates magnetic signal by driving load
current having a rising portion and a falling portion through the
inductive coil in a forward direction or in a reverse direction
with programmable load current rising and falling slopes to induce
recognizable back electromagnetic force at a receiver. It emulates
the magnetic strip data during the load current rising and falling
portions and reduces power loss during time periods without signal
transmission by controlling current slopes including pulse width
modulation (PWM).
In examples of the present disclosure, the method includes
providing diodes connected across each of the first, second, third
and fourth switches to facilitate free-wheeling of the current
corresponding to stored energy in the inductor coil during an OFF
period of the switches.
In examples of the present disclosure, the driving of the load
current through the inductive coil with the programmable load
current rising and falling slopes includes selectively and
repeatedly switching the pair of low side switches or the pair of
high side switches.
In examples of the present disclosure, the driving of the load
current through the inductive coil with programmable load current
rising and falling slopes by selectively and repeatedly switching
the pair of low side switches comprises turning the first switch in
a continuously ON state and repeatedly switching the fourth switch
between an ON state and an OFF state to drive the load current in
the forward direction with programmable load current rising and
falling slopes. It further comprises turning the second switch in a
continuously ON state and repeatedly switching the third switch
between an ON state and an OFF state to drive the load current in
the reverse direction with programmable load current rising and
falling slopes.
In examples of the present disclosure, the driving of the load
current through the inductive coil with programmable load current
rising and falling slopes by selectively and repeatedly switching
the pair of high side switches comprises turning the fourth switch
in a continuously ON state and repeatedly switching the first
switch between an ON state and an OFF state to drive the load
current in the forward direction with programmable load current
rising and falling slopes. It further comprises turning the third
switch in a continuously ON state and repeatedly switching the
second switch between an ON state and an OFF state for driving the
load current in reverse direction with programmable load current
rising and falling slope.
In examples of the present disclosure, the method further comprises
driving of the load current in forward direction with programmable
load current rising and falling slopes including repeatedly
switching the switches between an ON state and an OFF state
according to duty cycles of a first PWM control signal and a second
PWM control signal. It adjusts the duty cycle of the first PWM
control signal to set a rising portion of the forward load current
to a current limit in a positive first slope to induce a negative
back electromagnetic force lower than a negative reference voltage
in the receiver to recognize the induced back electromagnetic force
corresponding to a low pulse signal. It adjusts the duty cycle of
the second PWM control signal to set a falling portion of the
forward load current after reaching to the current limit in a
second slope opposite to the positive first slope and slower than
the first slope value to induce a positive back electromagnetic
force lower than a positive reference voltage in the receiver,
which can be ignored, to reduce the power loss during time periods
without signal transmission.
In examples of the present disclosure, the method comprises the
driving of the load current in the reverse direction with
programmable load current rising and falling slopes includes
repeatedly switching the switch between an ON state and an OFF
state according to duty cycles of the first PWM control signal and
the second PWM control signal. It adjusts the duty cycle of the
first PWM control signal to set a falling portion of the reverse
load current to the current limit in negative first slope to induce
a positive back electromagnetic force higher than the positive
reference voltage in the receiver to recognize the induced back
electromagnetic force corresponding to a high pulse. It adjusts the
duty cycle of the second PWM control signal to control a rising
portion of the reverse load current after reaching the current
limit in a positive second slope slower than the first slope to
induce a negative back electromagnetic force higher than the
negative reference voltage in the receiver, which can be ignored,
to reduce the power loss during the time periods without signal
transmission.
In examples of the present disclosure, the positive first slope is
selectively attained to provide load current rising time duration
sufficient to recognize the induced back electromagnetic force
signal in the receiver as a low pulse signal.
In examples of the present disclosure, the negative first slope is
selectively attained to provide load current falling time duration
sufficient to recognize the induced back electromagnetic force
signal in the receiver as a high pulse signal.
In examples of the present disclosure, the driving of the load
current through the inductive coil with programmable load current
rising and falling slopes includes an intermediate stage between
forward and reverse current driving. The load current in the
intermediate stage reduces to zero for better power efficiency.
In examples of the present disclosure, linearity of the load
current rising and falling slopes is controlled by changing the
duty cycle of PWM control signal. For linear slope, the PWM duty
cycle is varied. For non-linear logarithmic slope, the PWM duty
cycle is a constant.
In examples of the present disclosure, PWM switching frequency
during the rising and falling current slope intervals is much
faster than the input signal frequency to reduce current ripple in
the load current.
In examples of the present disclosure, the method may include pulse
frequency modulation (PFM) method including constant on-time
control and constant off-time control to control the load current
slopes.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an inscription of payment card data on a magnetic
stripe of the payment card and a waveform corresponding to the
magnetic stripe data as picked up by the payment terminal's card
reader while swiping of the payment card along with a digital
equivalent of the waveform.
FIG. 2A shows a circuit representation of the MST driver.
FIG. 2B shows an operation of the conventional MST driver switches
and corresponding load current waveforms in an inductive coil of
the MST driver.
FIG. 2C shows switching cycles of the MST driver switches,
corresponding load current waveform in the MST coil and an induced
back electromagnetic force (B.sub.emf) at card reader's
receiver.
FIG. 3 [from (i) to (iv)] shows an MST driver's switch driving
mechanism in a full bridge type switch configuration of an MST
driver in examples of the present disclosure.
FIG. 4 shows the MST driver's switching operation with the load
current waveform and the induced back electromagnetic force at the
receiver in examples of the present disclosure.
FIGS. 5a and 5b show comparison of power consumption in the prior
art MST driver and the disclosed MST driver.
FIG. 6 shows PWM switching of the high side switches 201, 202
comparing to the low side PWM switches in FIG. 4.
FIG. 7 shows the MST driver's switching operation with the load
current waveform and the induced back electromagnetic force at the
receiver including PWM in the rising current slope only in examples
of the present disclosure.
FIG. 8 shows the MST driver's switching operation with the load
current waveform and the induced back electromagnetic force at the
receiver including PWM in the falling current slope only in
examples of the present disclosure.
DETAILED DESCRIPTION OF THE INVENTION
An MST driver and a method for driving the MST driver are
disclosed. It is for low power, reliable transmission of emitted
signals of the MST driver by controlling the current slope of the
emitted signals using pulse width modulation (PWM) technique. The
MST driver is configured to transmit magnetic strip data including
streams of high or low pulses. The MST driver comprises a pair of
high side switches including a first switch and a second switch and
a pair of low side switches including a third switch and a fourth
switch. The switches are arranged in a full bridge type
configuration connected across a voltage source and a ground. An
inductive coil is connected across outputs of the full bridge type
configuration of the switches.
The MST driver further comprises a switch driver that is configured
to drive the pair of low side switches and the high side switches
under current slope control using PWM. It is for inducing
recognizable back electromagnetic force (B.sub.emf) at receiver end
and for emulating the magnetic strip data during load current
rising and falling portions through the inductive coil.
The induced back electromagnetic force B.sub.emf is a time
derivative of the load current through the inductive coil. The
induced B.sub.emf is a negative value to the load current. The
induced B.sub.emf is calculated by:
.times. ##EQU00006## where, L.sub.1 is the inductance value of the
inductive coil and
##EQU00007## is the time derivative of the load current through the
inductive coil.
##EQU00008## corresponds to the load current slope.
The switch driver of the MST driver includes a pulse width
modulator configured to generate a first pulse width modulation
control signal (PWM 1) and a second pulse width modulation control
signal (PWM 2). The switch driver drives either the pair of the low
side switches or the pair of the high side switches by selectively
and repeatedly switching between ON state and OFF state. It is to
drive load current involving a rising portion and a falling portion
through the inductive coil either in forward or in reverse
direction with programmable load current rising and falling slope
to generate a magnetic signal. It is for inducing the recognizable
back electromagnetic force at the receiver end and for emulating
the magnetic strip data during the load current rising and falling
portion.
A method using PWM controls on-time or off-time of the driver
switches to change the average current in the inductive load of the
MST driver. The controlling of the on-time or the off-time of the
MST driver switches enables the method to program the current slope
according to application requirement. The method can transmit the
signal more reliably and efficiently. The method illustrates how to
control or program the current slope stably through the PWM control
even in a condition with different power supply voltage and MST
coil.
FIG. 3 shows an MST switch driving mechanism in a full bridge type
switch configuration of an MST driver in examples of the present
disclosure. The MST driver comprises a pair of high side switches
201, 202 and a pair of low side switches 203, 204 arranged in a
full bridge type configuration connected across a voltage source
V.sub.M 208 and a ground and an inductive MST coil 206 having
inductance L.sub.1 and series resistance R.sub.1 207. Each of the
MST driver switches includes a respective diode (D1-D4) connected
across the switch 201, 202, 203, 204. It plays a role of
free-wheeling current path of stored energy in the inductive MST
coil 206 during the switch off period. In examples of the present
disclosure, the full bridge type switch configuration of the MST
driver comprises a first and a second high side low side
metal-oxide semiconductor field-effect transistor (MOSFET) pairs
connected in parallel between the voltage source and the ground.
Each high side low side MOSFET pair comprises a high side MOSFET
and a low side MOSFET connected in series with the drain of the
high side MOSFET connected to the voltage source and the source of
the low side MOSFET connected to ground, and the inductive MST coil
206 and the series resistance R.sub.1 207 connected between the
common (outputs) nodes of the two high side low side MOSFET
pairs.
The MST switch driving method as shown in FIGS. 3(i)-(iv) discloses
the driving of the load current involving a rising portion and a
falling portion through the inductive coil in a forward and in a
reverse direction with a programmable load current rising and
falling slope by selectively and repeatedly switching the pair of
the low side switches 203, 204 of the MST driver. Each switch is
driven by a separate driving signal different from each other.
In the MST driver, for driving the load current in the MST coil in
the forward direction the first switch 201 are continuously turned
ON and the fourth switch 204 repeatedly switches between ON and OFF
state. For driving the load current in the MST coil in the reverse
direction, the second switch 202 are continuously turned ON and the
third switch 203 repeatedly switches between ON and OFF state
according to duty cycles of PWM control signals.
FIG. 4 shows MST driver's switching operation with the load current
waveform and the induced back electromagnetic force at the receiver
end in examples of the present disclosure. The waveforms have a
same period P.sub.0 divided into 6 time intervals T.sub.1, T.sub.2,
T.sub.3, T.sub.4, T.sub.5 and T.sub.6. The time intervals T.sub.1
and T.sub.2 are corresponding to "forward switching". The time
intervals T.sub.4 and T.sub.5 are corresponding to "reverse
switching". First current slope (Slop1) during the time interval
T.sub.1 and T.sub.4 is much faster than second current slope
(Slop2) during the time interval T.sub.2 and T.sub.5.
Referring now to FIG. 3(i) and FIG. 4, during T.sub.1 time
interval, the first switch 201 is turned ON and the fourth switch
204 repeatedly switched between ON and OFF states based on a duty
cycle of the first PWM control signal PWM 1. The other two switches
202, 203 stay in OFF state. During the T.sub.1 interval, the load
current (I.sub.L) increases by using PWM1 method to a current limit
I.sub.L.sub._.sub.lim which limits the excessive load current.
The PWM1 makes the fourth switch 204 repeatedly switched between ON
and OFF state based on duty cycle (on-time/period). The duty cycle
increases to maximize current when it is close to
I.sub.L.sub._.sub.lim. The PWM1 controls rising of the forward load
current to a current limit in a first positive slope Slop1. The
Slop1 value is determined to ensure the induced negative back
electromagnetic force (-V.sub.fast) is lower than negative
reference voltage (-V.sub.r) in the receiver end. Therefore, the
receiver can recognize the induced back electromagnetic force
corresponding to a low pulse signal.
Another factor for successful transmission is that the Slop1
duration time T.sub.lo should be long enough to recognize the
induced back electromagnetic force in the card reader. In the MST
driver, the Slop1 is controlled by setting the duty cycle of the
PWM1. The instant load current during PWM control signal has a
small saw-tooth waveform ripple. But, the load current I.sub.L
waveform in FIG. 4 shows the average value of the load current
during PWM.
When the load current reaches the I.sub.L.sub._.sub.lim during
T.sub.1 time interval, it begins to decrease and has a second
current slope (Slop2) controlled by second PWM control signal PWM2
during the T.sub.2 interval. In the T.sub.2 interval as shown in
FIG. 3(ii) and FIG. 4, the fourth switch 204 still switches between
ON and OFF state but its duty cycle is different from T.sub.1. The
PWM2 makes the second slope (Slop2) opposite to the Slop1 and much
slower than Slop1 value. Therefore, the induced positive back
electromagnetic force (V.sub.slow) becomes much lower than the
positive reference voltage (V.sub.r) in the receiver end and is
ignored by the receiver. This operation reduces the power loss
drastically comparing to the prior art. In conventional operation,
the load current is fixed to a constant value for the T.sub.2
interval and consumes lots of power loss (Iout*VM) without
conducting work.
The Slop2 and the end current level are dependent on the coil
inductance value (L.sub.1), the peak current level
(I.sub.L.sub._.sub.lim), the period (P.sub.0), and card reader's
receiver reference voltage level (V.sub.r). The end current level
may or may not reach zero level. The Slop2 can be controlled by
setting the duty cycle of the PWM2.
If the load current is decayed completely before the T.sub.3
interval of FIG. 3(ii), during the T.sub.3 interval, the load
current is zero and the fourth switch 204 as well as the second
switch 202 and the third switch 203 are turned off. The first
switch 201 may be on or off. If the load current in the T.sub.3
interval doesn't exist and is skipped, the T.sub.4 interval starts
after T.sub.2 interval. The longer the T.sub.3 time, the better the
power efficiency.
The T.sub.4 interval as shown in FIG. 3(iii) and FIG. 4 is the same
as the T.sub.1 interval except that the load current directions are
in opposite directions. In the T.sub.4 interval, the first switch
201 and the fourth switch 204 are turned off. The second switch 202
is turned on continuously. The third switch 203 is switched between
ON and OFF state according to the PWM1. In the T.sub.4 interval,
the first slope Slop1 is negative and the induced back
electromagnetic force is positive. The induced positive back
electromagnetic force (+V.sub.fast) should be higher than V.sub.r
and the time (T.sub.hi) should be long enough, so that the receiver
can identify the high pulse signal.
The T.sub.5 interval of FIG. 3(iii) and FIG. 4 is the same as the
T.sub.2 interval except that the load current directions are in
opposite directions. During the T.sub.5 interval, the first switch
201 and the fourth switch 204 are turned off continuously. The
second switch 202 is turned ON. The third switch 203 is switched
between ON and OFF state according to the PWM2. The second slope,
Slop2, is positive and the induced back electromagnetic force
(-V.sub.slow) is negative comparing to that in the T.sub.4 interval
and is higher than -V.sub.r, which can be ignored at the receiver
end. In examples of the present disclosure, all the operations of
T.sub.5 are the same as T.sub.2 except that their directions are
opposite.
The T.sub.6 interval of FIG. 3(iv) and FIG. 4 is the same as the
T.sub.3 interval.
In the MST driver's switch driving operation as described above,
when the first 201 and the fourth 204 switches or the second 202
and the third 203 switches are turned on, the load current I.sub.L
level increases for switch on time (t.sub.on) period by Eq. 1 and
Eq. 2.
.DELTA..times..times..times..times..times..times..times.
##EQU00009## V.sub.drop1=I.sub.L*(R.sub.1+R.sub.on1,2+R.sub.on4,3)
Eq. 2
where V.sub.M is the power supply voltage. L.sub.1 is the
inductance value of the MST coil. R.sub.1 is the series resistance
of the coil. R.sub.on1,2 is the on-resistance of the first switch
201 or the second switch 202 in the high side. R.sub.on4,3 is the
on-resistance of the fourth switch 204 or the third switch 203 in
the low side.
When the first 201 or the second 202 switch is turned on and the
fourth 204 and the third 203 switches are turned off, the I.sub.L
current decreases for switch off time (t.sub.off) period by Eq. 3
and Eq. 4. This period is called as a freewheeling.
.DELTA..times..times..times..times..times..times..times..times..times.
##EQU00010## V.sub.amp2=I.sub.L*(R.sub.1+R.sub.on1,2+R.sub.on2,1)
Eq. 4
where V.sub.F2,1 is the forward voltage of D2 or D1 and R.sub.on2,1
is the on-resistance of the second 202 or the first 201 switch in
the high side.
FIG. 5a shows a power loss reduction of the MST driver. In a prior
art method, the steady state of the load current consumes a lot of
power without conducting work because the signal transmission
happens during the transient period of the load current. Therefore,
the longer period P.sub.0 is, the bigger power loss is consumed and
the higher temperature is reached. In a battery system, it will
make the battery re-charge more frequently.
In FIG. 5b, the load current is decaying during the T.sub.2 and
T.sub.5 intervals of FIG. 4 and can reach zero level. Therefore, if
T.sub.3 and T.sub.6 intervals are long, the average power loss,
P.sub.loss.sub._.sub.avg, becomes at least lower than a half of
that in the conventional MST driver.
In the prior art method, the slope changing quickly in the
beginning stage of transition is out of control due to the
freewheeling operation and the high voltage (V.sub.M+2VBE) applied
to the inductor. It generates lots of high frequency noise
including EMI which can cause many side effects. However, since the
disclosed MST driver can control the current slope, optimal
condition between the performance and the noise can be
achieved.
The MST drive includes the linear and non-linear (logarithmic)
rising and falling in the load current using PWM. According to the
PWM duty cycle the current slope can be controlled to be linear or
non-linear. The rising and falling current slopes of FIG. 4 are
linear. For the linear slope, the PWM duty cycle is varying.
However, if the PWM duty cycle is constant, the rising and falling
current slopes becomes non-linear (logarithmic).
The PWM switching frequency during the fast/slow rising/falling
current slope intervals, T.sub.1, T.sub.2, T.sub.4 and T.sub.5, is
set much faster than the input signal frequency, 1/P.sub.0, to
minimize current ripple in the load current.
The absolute value of |Slop1| and |Slop2| are determined by
V.sub.r, -V.sub.r, T.sub.hi, and T.sub.lo for successful signal
transmission. |Slop1| is designed for B.sub.emf to induce the
receiver to generate a voltage signal higher than V.sub.r. |Slop2|
is adjusted for B.sub.emf to induce the receiver to generate a
voltage signal lower than V.sub.r that can be ignored. |Slop1| is
much higher than |Slop2| with reference to V.sub.r and -V.sub.r.
The control purpose of the |Slop1| is for more successful data
transmission and the |Slop2| is for power loss reduction.
FIG. 6 shows the PWM switching of the high side switches 201, 202
(to be compared with the low side PWM switching of FIG. 4). It
shows almost same result as the low side PWM switching.
The PWM method of the present disclosure can be replaced by a pulse
frequency modulation (PFM) method including constant on-time
control and constant off-time control. In examples of the present
disclosure, the load current slope can be controlled by the PFM
method instead of PWM method to get similar waveforms of FIG.
4.
FIG. 7 shows the MST driver's switching operation with the load
current waveform and the induced back electromagnetic force at the
receiver end with PWM in the rising current slope in examples of
the present disclosure. The switch driver includes a pulse width
modulator being configured to generate only the first PWM control
signal, PWM1 of Slop 1. It is only for more successful data
transmission.
FIG. 8 shows the MST driver's switching operation with the load
current waveform and the induced back electromagnetic force at the
receiver end with PWM in the falling current slope in examples of
the present disclosure. The switch driver includes a pulse width
modulator being configured to generate only the second PWM control
signal, PWM2 of Slop 2. It is only for power loss reduction.
Those of ordinary skill in the art may recognize that modifications
of the embodiments disclosed herein are possible. For example, time
intervals may vary. Other modifications may occur to those of
ordinary skill in this art, and all such modifications are deemed
to fall within the purview of the present invention, as defined by
the claims.
* * * * *